1. Field of the Invention
The present invention relates to a water based ink set for ink-jet recording and to an ink-jet recording method.
2. Related Art of the Invention
An ink-jet recording system has been widely used for color-printing digital color image data. In such an ink-jet recording system, a water based ink set for ink-jet recording has been employed as ink-jet inks. The water based ink set comprises a yellow ink, a magenta ink and a cyan ink which are each prepared by dissolving a water soluble dye into a mixed solvent of water and water soluble organic solvents. In some cases, in order to print text data such as alphabet character data and Chinese character data, a dye based black ink is added to the water based ink set for ink-jet recording. Further, in order to implement vivid color reproducibility, it has been proposed to constitute an ink set with seven water soluble dye based color inks including a yellow ink, a magenta ink, a cyan ink, a red ink, a green ink, a blue ink and a black ink (see Japanese Patent Application Laid-Open No. 2002-241661).
When employing such an ink set, however, the number of nozzles of an ink-jet head must be increased, resulting in the problem of cost increase.
Also, when an ink set is composed of dye based inks only, there is a problem that feathering is noticeable in text. Feathering is fine scale blurring caused by the presence of fibers and spaces in a recording material such as recording paper when ink lands on the recording material to penetrate thereinto.
Recently, in addition to vivid color reproducibility, sharp text print quality has been desired in which blurring such as feathering is suppressed as much as possible. Print quality close to photo image quality has also been desired in which excellent lightfastness and ozone resistance are implemented.
In order to implement sharp text printing, Japanese Patent Publication Laid-Open No. 2001-354886 discloses an ink set for ink-jet recording comprising, in addition to the three primary color inks (yellow, magenta and cyan inks), three additional color inks (black, red and green inks), all of which are composed of pigment based inks.
However, when all of the inks of the ink set are composed of pigment based inks, despite that the additional black, red and green inks are employed together, the color reproducibility is not satisfactory as compared to that of an ink set composed of dye based three primary color inks.
Therefore, in order to implement both sharp print quality without feathering in text and vivid color reproducibility close to silver halide photograph in a color image, a commercially available ink set employs a pigment as a coloring agent for a black ink in which the print quality of text is of importance and employs water soluble dyes as coloring agents for color inks (including yellow ink, magenta ink and cyan ink) in which color reproducibility is of importance.
However, when text is printed by use of an ink set having the abovementioned ink configuration, problems have arisen in text printed in red and in blue, since red color and blue color besides black color are frequently used for emphasis, correction and the like in the actual text.
That is, the red color is expressed by superposing the dye based yellow ink and the dye based magenta ink, and the blue color is expressed by superposing the dye based magenta ink and the dye based cyan ink. Because of this, ink misplacement caused by a landing error of the superposition occurs in each of the text printed in red and in blue to cause the sharpness of the text to deteriorate. In addition, since two kinds of the dye based inks are employed for expressing the red color and the blue color as described above, the amount of the inks employed for expressing each of the colors is about twice as much as the amount of-the black pigment ink employed for printing text in black, thereby causing a problem that feathering tends to occur.
In addition, the text printed in red or in blue also has problems of low water resistance, low lightfastness and low ozone resistance. More specifically, when the water resistance of the inks is low, for example, blurring of a text document printed in an office is likely to occur due to sweat of a human hand or water such as a water drop from a drink, and also migration (i.e., blurring caused by moisture) tends to occur.
When the lightfastness is low, the fading of the red color and the blue color of the text posted for a long period of time occurs due to exposure to illumination light or sunlight, causing the text to have a short shelf life. Particularly, the lightfastness of the dye based magenta ink, which is employed to express the red color together with the dye based yellow ink, is lower than those of the yellow ink, the cyan ink and the black ink. Therefore, the color balance of the image will be disrupted after the color fading, causing the redness of the image to degrade.
Moreover, color fading of an image caused by various gases present in air (particularly by ozone) has recently been paid attention, and thus high ozone resistance has been required. However, the ozone resistance of the magenta ink employed for expressing red color and the cyan ink employed for expressing blue color is lower than those of the yellow ink and the black ink. Therefore, the color balance will be disrupted after the color fading due to aging, causing the redness and the blueness of the image to be degraded. Consequently, yellow color will mainly remain on a recording material after the fading of the magenta ink and the cyan ink due to aging, causing the yellowness of the image to be enhanced. Such disruption of the color balance after the color fading due to aging gives an unnatural impression, and this has been a serious problem in improving print quality.